It seems the harder scientists listen to animals, the more they end up eavesdropping on their conversations. Take butterflyfishes—flamboyantly colored, hand-sized denizens of coral reefs, known for their monogamy, gregariousness, and fierce territoriality. New research, reported Wednesday at the joint meeting of the Acoustical Society of America and Acoustical Society of Japan, shows that butterflyfishes make a variety of sounds to communicate among themselves. The fish may have evolved unique anatomy to enhance their use of sound, the researchers say.
All fish have internal "ears," air-filled swim bladders sensitive to sound waves, and "lateral line" sense organs that detect motion in surrounding water. However, only in one genus of butterflyfishes are these body parts connected—a discovery made some years ago. Scientists have speculated that the unusual anatomical arrangement is involved in sound perception, but no one knew what role, if any, sound plays in butterflyfish lives.
To find out, marine biologist Tim Tricas of the University of Hawaii, Manoa, and colleagues dove to a Hawaii reef and located several pairs of banded butterflyfish (Chaetodon multicinctus) observed maintaining feeding territories. In multiple experiments, the researchers placed one pair in a glass bottle and positioned the bottle inside another pair's territory for up to 40 minutes. The results, recorded by a video camera and underwater microphone, revealed territory defenders aggressively charging the intruders, while making rapid, sound-generating moves, such as flicking and erecting their fins, "jumping," and turning. In response, the bottled fish grunted repeatedly. Only paired fish grunted—not single individuals—so Tricas suspects grunts are distress signals to mates.
In another experiment, the researchers injected tiny amounts of petroleum jelly into the swim bladders of butterflyfish pairs, interrupting the transmission of sound through the swim bladder to their ears and lateral lines. Treated pairs swam significantly closer together than untreated pairs, suggesting that their ability to sense any sound from each other was compromised. Given butterflyfishes' unusually social nature, they may have evolved their novel anatomical connections to facilitate communication, Tricas adds.
The connections between sound production and social behavior had gone unnoticed despite intense field study of this fish group over several decades, says Jacqueline Webb, a marine biologist at the University of Rhode Island. The findings underscore the need "to consider the sound dimension of reef communities when contemplating not only the behavior and ecology of animal populations, but the increasing impacts of human noise on the underwater environment," she adds.